The central objective of this program continues to be the systematic immunogenetic definition and analysis of components of the cell surface that are programmed for selective expression on cells of particular lineages. While the emphasis is on lymphocytes, not only because of their immunological relevance but also because of their unique accessibility to serological and functional analysis, growing importance attaches to systems which are also manifested in other hematopoietic sublineages. The Ly-5 system especially emphasizes this trend, for we now have tentative serological evidence suggesting that the molecular diversity of Ly-5, which accords with hematopoietic lineage, is due at least in part to genetic complexity at the Ly-5 locus (which may therefore represent a superfamily of considerable interest) rather than to alternative processing of a single Ly-5 product. This will be one focus of priority in the coming year. Among four new alloantigenic systems to be studied in detail is GM-1 (granulocyte-monocyte-1, identified by monoclonal alloantibody), sufficiently defined to be named. Expression of GM-1, it appears, may be restricted to the (neutrophil) granulocyte-monocyte branch of hematopoiesis. It is remarkable that the strain distribution of GM-1 alloantigen (only one allele is known so far) is completely concordant with that of Ly-6.2, including an Ly-6 congenic strain. Since Ly-6 and GM-1 cannot be identical, we shall be studying the GM-1 locus in the likely context of a superfamily of genes (chromosome 9) governing a range of lineage-related cell surface glycoproteins (about which there is already some evidence in the literature). A second of the four systems mentioned, again defined by monoclonal antibody and restricted to hematopoietic lineages, is notable for the recognition of two molecular forms, one of approximately 190,000 molecular weight and the other of approximately 90,000, not related to any previously known system or cell surface component. Despite the gross molecular structural resemblance to Mac-1 and LFA-1, defined by xenoantibodies, peptide mapping indicates that these are different systems. Appropriate backcrosses set up for general genetic purposes will be continued to derive congenic strains for finer definition of these new systems in terms of program and function.